The invention relates to deep or long wells, in particular oil wells.
As an oil well advances, from time to time one or more tubes have to be added at the well head. For this reason, such a well comprises a vertical support structure on the surface which is known as a derrick.
In general, the derrick and the equipment it contains, in particular the rotary drive system, will be known herein as the “well head equipment”.
As will be seen in more detail below, the area of the well head equipment is a cramped space. The derrick carries a mechanical system which can hold the drill string as well as lift and drop it. When pulling the drill string, the upwards vertical excursion is ten meters or more. This is also the case for the downwards vertical excursion during drilling.
The well head equipment also includes the rotary drive for the string, for drilling, and also the system that can break out and make up one or more tubes (or other equipment) onto the formed string. To this is added a system for injecting and recovering drilling mud which in particular actuates the drill bit. Finally, various types of safety systems are necessary.
The search is currently on to make the drill string communicative, so that information can be exchanged between the top and bottom of the well or between intermediate equipment inserted in the string. To this end, within the string, each tube is provided with communication couplers at its ends and an electrical connection between those couplers.
At the top of the well, it is helpful to pass data between the string, which rotates and is displaced vertically, and fixed electronic equipment on the surface. The connection between the string and this surface electronic equipment is known herein as the “surface interface”.
Selecting this surface interface connection is critical. If this connection is interrupted, the communication equipment provided in the string itself becomes useless and all of the information and commands it relies upon is lost.
A number of solutions can be envisaged. Of these, few are actually applicable in practice, due to the many constraints placed upon it, in particular the cramped environment of the well head equipment.
As an example, US 2010/0214121 describes a drill string provided with a communication device comprising a single transmitter capable of carrying out wireless transmission with one or more fixed “coordinators”. Said coordinators are in fact receiving antennae located at the well head. The envisaged transmission conforms to IEEE standard 802.15.4, which allows data transfer at a fairly low rate.
The communication device in question uses control or marker signals originating from the coordinators in order to determine the best available coordinator and/or, for one coordinator in particular, the best moment to transmit data to it.
Data transmission is limited to a portion of the rotational gate of the string, typically an arc of 120.degree., or to certain time periods. This is intended to minimize the energy consumed for data transmission.
It is then necessary to adjust the transmitter on the coordinators to determine the time periods which are favorable for data transfer: once a valid coordinator has been detected, the rate of rotation of the string is used to determine a succession of transmission time periods which correspond to successive passes of the transmitter in the vicinity of the coordinator in question.
The device known from US 2010/0214121 suffers from a number of disadvantages.
As has been seen, the rate of rotation of the string must be known, but it is likely to vary with time, for example as drilling problems crop up. Typically, supplemental sensors have to be installed in the string, which causes problems with integration thereof and electrical supply thereto.
Further, for high rates of rotation, little time is available to carry out data transfer properly.
It also appears that the device in question only accommodates a fairly reduced transfer rate, which may prove to be insufficient, primarily because of recent improvements in the field of sending information from the well bottom.
Finally, it is not a simple matter of multiplying the number of coordinators, i.e. the receiving antennae, since as has been seen above, the environment of the well head equipment is already very cramped and filled with elements that are vital to the drilling operations.
US 2010/0224409 A1 describes a wear insert or saver sub which is used to connect the drill string to the drive system. Said wear insert is equipped with antennae which allows wireless data transmission with a surface antenna produced in the form of a parabolic antenna. According to US 2010/0224409 A1, transmission/reception of data can be carried out in practically any direction, in particular over 360.degree. around the wear insert, which means that communication should be established even when the wear insert is driven in rotation or displaced in any manner. The function of said antennae is not explained in more detail. The idea appears to be to irradiate the antennae array around the wear insert as widely as possible in all directions so that the parabolic antenna can practically always capture data transmitted by the insert in question. The system in US 2010/0224409 A1 does not overcome certain problems which arise in practice in the art, such as the energy consumption of the onboard electronic elements or the cramped space in the well environment, to mention just a few.
The present invention will improve the situation.